The present invention relates to a wound capacitor preferably formed from layers of metalized film and metal electrodes. The capacitor has improved heat transfer performance that reduces internal thermal stress and permits increased current-carrying capacity.
A metalized film capacitor is formed from layers of metallic film on a dielectric substrate. A suitable metal, such as aluminum, is vapor deposited on the substrate. The thickness of the metallic film is typically in the range of 100 to 400 angstroms. The dielectric substrate is generally a thermoplastic resin, such as polypropylene. The thickness of the substrate is significantly greater than the thickness of the film. Two metalized film layers are convolutely wound together to form to form he capacitor. In this configuration, the dielectric substrates separate the two metalized film layers, which form the electrodes of the capacitor.
Film and foil (film/foil) capacitors are formed from four alternating layers of a dielectric and a metallic foil. The two layers of metallic foil form the electrodes of the capacitor. The thickness of the foil is typically in the range of 0.0005 to 0.0025 centimeters (cm). In this configuration, the dielectric layers separate the two metal electrodes. By extending the longitudinal ends of the two metallic films layer beyond the ends of the dielectric layers, at opposite axial ends of the capacitor, an extended foil capacitor is formed. A conductive metal end-spray can be applied to the extended whorl of metallic foils at each axial end of the capacitor to provide for the connection of terminals to the capacitor.
A hybrid of a metalized film and a film/foil capacitor is known as a metalized film and foil (metalized film/foil) capacitor. FIGS. 1(a) and 1(b) show typical cross sectional layering for series-wound metalized film/foil capacitors known in the art. In FIG. 1(a), a metalized film layer, composed of a metalized film 152 on a dielectric substrate 153, is wound between a dielectric layer 130 and a layer composed of two separate strips of metallic foil 120 and 122. The strips of metallic foil form the two metal electrodes of an extended foil capacitor. FIG. 1(b) illustrates a common variation of the hybrid capacitor in FIG. 1(a). For the configuration shown in FIG. 1(b), two metalized film layers are wound with their metallic films in contact with each other to double the thickness of the metalized film conductor. The space between the inner longitudinal ends of the foils in FIGS. 1(a) and 1(b) is referred as the margin. As show in these figures, the margin has a width 190. Minimum cross sectional margin widths are primarily driven by voltage withstand values determined from the working voltages for the wound capacitor. Typical widths for the margin at 600 volts ac are on the order of 0.4 cm.
Internal heat is generated in a capacitor in two ways, namely dielectric heating and conduction losses. Dielectric heating is linearly proportional to the operating frequency of the capacitor and proportional to the square of its operating voltage. Conductive (I2R) losses are proportional to the square of the current carried by the capacitor. Consequently, for high frequency and high current capacitors, the amount of internal heat generated is a significant and limiting factor for the use of a metalized film/foil capacitor. The capacitor temperature due to internal heating plus the ambient temperature should not exceed the maximum allowable operating temperature for the selected dielectric, or it is possible that the capacitor will open or, in some cases, rupture.
In metalized film/foil capacitors, the electrodes formed by the metalized film have a significant resistance in comparison with the resistance of the metallic foil metalized film. Consequently, significant conduction losses are generated in the conductor formed by the metalized film. The metallic foils have a relatively high value of thermal conductivity and serve as an efficient conductor of heat from the interior of the wound capacitor. Conversely, the dielectric substrate typically is a very poor thermal conductor. Therefore, the metallic foils play an important part in the dissipation of heat from the dielectric losses and the conduction losses in the metalized film.
In a series-wound capacitor with electrodes formed from extended metallic foils, the foils act as an effective heat sink in contact with the dielectric substrate except for the region formed by the margin. Current flows across the margin through the metalized film. Internal heat will build up in the region formed by the metalized film bridging the margin. In the prior art capacitors shown in FIGS. 1(a) and 1(b), the nearest surface of metallic foil from the center of the metalized film bridging the margin is equal to half of the margin""s width. This is more clearly appreciated when looking at a cross section of multiple coils of the layers, such as in FIG. 1(c), for two coils of the prior art capacitor in FIG. 1(b). For the typical margin width of 0.4 cm noted above, the nearest distance to the surface of a metallic foil from the center of the metalized film bridging the margin will be 0.2 cm.
The present invention addresses the problem of internal heat buildup by using two parallel capacitors wound together with adjacent margins being offset so that all (including the center) of the metalized film bridging the margin is no further away than approximately the thickness of one dielectric substrate from the surface of a metallic foil. For the typical 0.001 cm thickness of a dielectric substrate, the nearest distance to the surface of a metallic foil from all of the metalized film bridging the margin will be 0.001 cm. This is a significant decrease in the distance to a metallic foil that will act as a heat sink for heat transfer from the metalized film bridging the margin. This results in a significant increase in the amount of current that can be carried by a capacitor using the same amount of materials as that for the prior art capacitors shown in FIGS. 1(a) and 1(b).
In its broadest aspect, the present invention is an axially wound capacitor formed from at least two composite layers. Each composite layer is formed from component layers. In the preferred embodiment of the invention, one electrode layer and two metalized film layers form the minimum component layers. Each electrode layer is formed from two metal electrodes. There is a space, or margin, between the adjacent longitudinal ends of the two metal electrodes. The two metal electrodes have their outer longitudinal ends disposed at opposite axial ends of the capacitor. The longitudinal ends of all metal electrodes at each axial end of the capacitor are electrically connected together to form the two external electrical connection points for the capacitor. Each of the two metalized film layers of each composite layer is formed from a metalized film on a dielectric substrate. The metalized film layers for the two substrates are disposed adjacent to each other. All composite layers that make up the capacitor are similarly oriented to each other with the exception that the margins in the electrode layers of adjacent composite layers are offset from each other.
Equal widths of diagonally opposed metal electrodes in adjacent electrode layers can be provided. The width of the margins in all electrode layers can be equal, and the margins can be offset in adjacent electrode layers by the width of a margin. In alternate embodiments of the invention, a dielectric layer can replace one of the two metalized film layers in one or more of the composite layers that form the capacitor. Also, a single metalized film layer can replace the two metalized film layers in at least one composite layer of the capacitor when its metalized film is placed adjacent to a dielectric layer, or the dielectric substrate of a metalized film layer. A dielectric layer can be placed between one or more adjacent composite layers.
In another aspect, the present invention is a wound capacitor formed from the a plurality of layers. The first layer is composed of first and second metal electrodes. There is a space, or first margin, between the adjacent longitudinal ends of the first and second metal electrodes. The second layer consists of a metalized film layer that is composed of a metalized film on a dielectric substrate. The second layer has its dielectric substrate facing the first layer. The third layer is a second metalized film layer that has its metalized film placed adjacent to the metalized film of the second layer. The fourth layer is composed of third and fourth metal electrodes. The electrodes are oriented so that the first and third metal electrodes have their outer longitudinal ends at the same axial end of the capacitor. There is a second margin between the third and fourth metal electrodes. The second margin is offset from the first margin. The fifth layer is a third metalized film layer that has its dielectric substrate adjacent to the fourth layer. The sixth layer is a fourth metalized film layer that has its metalized film adjacent to the metalized film of the fifth layer. The half-widths of the first and second margins are greater than the thickness of the first, second, third, or fourth metalized film layers. Equal widths of the first and fourth metal electrodes, and of the second and third metal electrodes can be provided. The widths of the first and of the second margins can be equal, and the margins can be offset from each other by the width of a margin. In alternate embodiments of the invention, at least one of the four metalized film layers can be replaced by a dielectric layer.
In another aspect, the invention is a method of manufacturing a wound capacitor by simultaneously winding layers of substantially equal lengths around a core. The first layer is composed of first and second metal electrodes. They are wound with a space, or first margin, between their adjacent longitudinal ends. The second layer consists of a metalized film layer that is composed of a metalized film on a dielectric substrate. It is wound with its dielectric substrate facing the first layer. The third layer is a second metalized film layer that has its metalized film placed adjacent to the metalized film of the second layer. The fourth layer is composed of third and fourth metal electrodes. The electrodes are oriented so that the first and third electrodes have their outer longitudinal ends at the same axial end of the capacitor. A second margin is formed between the third and fourth metal electrodes. The second margin is offset from the first margin. The fifth layer is a third metalized film layer that has its dielectric substrate adjacent to the fourth layer. The sixth layer is a fourth metalized film layer that has its metalized film adjacent to the metalized film of the fifth layer. In alternative embodiments of the invention, the layers may be reverse wound around the core. During manufacture, the half-widths of the first and second margins are maintained with a length greater than the thickness of the second, third, fifth, or sixth layers. The core, if electrically non-conductive, may be retained in the capacitor. Alternatively,-the core may be removed after winding the capacitor. If the core is removed, the wound capacitor may be flattened.
The wound capacitor can be manufactured with equal widths for the first and fourth metal electrodes, and for the second and third metal electrodes. Furthermore, the wound capacitor can be manufactured with equal widths for the first and second margins, and the margins can be offset from each other by this equal margin width. The wound capacitor can be manufactured so that the outer longitudinal ends of the four electrodes extend beyond the outer longitudinal ends of the metalized film layers. With this arrangement, the capacitor can be further processed by metal spraying the axial ends of the capacitor to form a common electrical connection between the first and third metal electrodes at one axial end of the capacitor, and between the second and fourth metal electrodes at the other end of the capacitor. A coating of solder can be provided over the ends of the capacitor. Oil can be inserted into the windings of the capacitor by scoring the solder to the outer longitudinal edges of the metalized film layers and injecting the oil into the windings. Electrical terminals can be attached to the axial ends of the capacitor to provide a means for connecting the capacitor to an external circuit. In alternative manufacturing processes, at least one of the metalized film layers can be replaced by a dielectric layer. In these embodiments, an electrode layer, a metalized film layer, or a dielectric layer may be the first layer wound on the core.
These and other aspects of the invention will be apparent from the owing description and the appended claims.